Stents and other metallic implants can cause a partial shielding of a radio frequency (RF) field by the Faraday Effect. In essences, the stent acts like a “Faraday Cage” that prevents the RF field from penetrating to the interior of the stent. Because stents are not ideal but only partial Faraday cages, a small percentage of the RF field still is able to penetrate to the interior, however not enough to cause enough spins to flip over and give a reasonable visibility.
One approach to achieving the reasonable visibility would be to raise the energy of the RF field (the flip-angle that stands for the duration of the RF-pulse) to such high levels that enough energy remains after passing through the partial stent shield for visualization. Unfortunately, taking this approach will cause the tissue of the body to be heated to unacceptable levels.